The creation of wireless and mobile devices depend upon various key requirements, such as minimizing power usage. Power carries the heavy burden of impacting battery life on these portable devices. Maximizing the amount of battery life is very important while being able to maintain performance and minimize cost. Minimizing the use of power during a read operation is a disadvantage that several of the prior art ROM memory array have.
During a read operation a word line may be turned on to activate a bit cell to be read. If the bit cell being read is programmed as a logical zero, then the bit line discharges through the bit cell transistor. If the bit cell being read is programmed as a logical one, then the bit line should remain high. Sub-threshold leakage and Source line to bit line coupling can prevent a bit line from staying high.
Sub-threshold leakage may occur in unselected/not read bit cells programmed as zero's that are coupled between the same bit line and source line. The bit cells that share a common bit line and source line may have leakage current from the high-voltage bit line through the unread bit cells to the low voltage source lines even though the unread or unselected bit cells do not have a signal to turn on the gates of those bit cells. Alternatively, even if no transistor exists in an unread bit cell location such as in diffusion programming, then that unread bit cell location still may lose part of its permanent charge during a read operation on a neighboring bit cells, through the charge leaking from the bit line through its neighboring cell. The cumulative affect of large numbers of unselected transistors in a column of bit cells, such as 1,024, can cause a memory location with or even without a transistor at that bit cells location to lose part of its permanent charge during a read operation on the neighboring bit cells.
Even without leakage current, bit line coupling, electrical interference passing between the closely formed source line and bit line may occur from the high-voltage bit line to the low voltage source line. Source line to bit line coupling can cause a bit line to drop its voltage level. Periodically a bit line may be recharged and consume battery power to make up for this cumulative drop in voltage.